Some conventional pressure transducers rely on the deflection of a precisely machined diaphragm to activate corresponding changes in strain in a highly pretensioned vibrating wire attached to the diaphragm. The force applied by a pressure p.sub.1 to which the transducer is exposed is absorbed in the diaphragm, and the electromagnetically excited vibrating wire only measures the diaphragm's response to that force. Because the diaphragm is deformationally much stiffer than the wire, deformation in the diaphragm controls the deformation in the wire. A change in the deformation of the wire in turn corresponds to a change of tensile stress in the wire which causes an electromagnetically detectable change in the resonant vibrating frequency of the wire. Such transducers commonly encounter problems of nonlinearity over the range of expected measurement values, in addition to other problems such as long-term wire "creep" or pretension loss, crimp slippage or deformation, and necessity of the temperature compensation.
Moreover, the typical reverse direction in which tensile stress in the vibrating wire is relieved (that is, an increase in p.sub.1 causes a decrease in the tensile stress in the pretensioned wire) compounds all of the above problems. The diaphragm of such a transducer has a controlled axial mechanical deformation response, when subjected to an external pressure p.sub.1 that is different from the internal reference pressure. Pressure measurement ranges may be changed only by changing the geometry or the thickness of the diaphragm. A more detailed analysis of this type of pretensioned vibrating wire transducer and its relative shortcomings and advantages with respect to the art is presented in commonly owned, copending application by Clements, Ser. No. 07/250,179, filed Sep. 28, 1988, now issued Jul. 3, 1990 as U.S. Pat. No. 4,938,068.
Other known pressure transducers employ a pressure deformable bellows to respond to a differential between a pressure p.sub.1 that is external to the transducer but which is applied internally to the bellows, and a reference pressure p.sub.0 external to the bellows. By attaching one end of an elastic vibrating wire or strip to a movable end of the bellows so that an increase in pressure p.sub.1 results in a direct increase in tensile stress in the wire, many of the difficulties encountered in the above described transducer structure are ameliorated. Unlike the diaphragm, the bellows is employed to directly convert the difference between p.sub.1 and p.sub.0 into a simple axial mechanical force that is transferred to a vibrating member. Also, the bellows typically has much less deformational stiffness than the vibrating member. The vibrating member is typically of a magnetically interactive material such as steel wire or a thin steel strip, and electromagnetic exciting and sensing means are associated with the vibrating member in a manner such as is summarized in Clements.
In these known transducers, the bellows is employed as the pressure chamber, with p.sub.1 being internally applied to the bellows, and is typically surrounded by a reference pressure chamber having therein a reference pressure p.sub.0. Various means and methods are also known for connecting the vibrating member to the movable end of the bellows so that an increase in pressure inside the bellows will increase the tensile stress in the vibrating wire.
For example, commonly owned copending application by Clements, referred to above, discloses a pressure transducer employing an external saddle type linkage on the outside of an expandable bellows to translate the axial mechanical force in the expanding bellows into an axial force in the vibrating member. But while Clements represents a significant improvement over previous designs, Clements nonetheless leaves unresolved certain problems of bellows stability and linear response in the pressure transducer.
In the method and apparatus disclosed and claimed by Clements, a differential in pressure .DELTA.p between a pressure p.sub.1 internal to the bellows and a reference pressure p.sub.0 surrounding the bellows acts to impart motion to a movable end of the bellows with respect to a fixed end of the bellows. Since p.sub.1 applied internally to the bellows acts symmetrically, any motion in the movable end of the bellows would tend to be perfectly uniaxial (that is, the bellows would have, for practical purposes, but a single axis of freedom of movement), but only if the bellows were mechanically unrestricted in its response to .DELTA.p. However in Clements, and in other known pressure transducers employing a pressure deformable bellows, the linkage between the movable end of the bellows and the vibrating member create together a mechanical system in a constrained state. Thus in Clements, for example, a positive .DELTA.p, which acts symmetrically to tend to increase the distance between the movable end of the bellows and the fixed end of the bellows, is opposed by the resistance of the mechanical linkage and the vibrating member to deformation. The force resulting from deformation of the vibrating member itself and the mechanical linkage to the movable end of the bellows tends to hold the bellows in a compressed state, and in fact applies a countervailing mechanical compressive force to the bellows to achieve equilibrium.
This compressive mechanical force is applied to the bellows in a typically unsymmetrical way and, unlike the influence of .DELTA.p which has no destabilizing influence on the bellows, the mechanical force causes a destabilizing influence on the bellows which tends toward a phenomenon of "buckling". Stability of the bellows in this compression mode depends on the stiffness of the bellows and the magnitude of mechanical force acting on the bellows material for equilibrium under the pressure applied internally to the bellows. The stability conditions of the bellows are described by Euler theory. For any given pressure p.sub.1, lower bellows stiffness results in an increase in the tendency for buckling deflections of the bellows. Thus in known pressure transducers the expected increase in linearity of transducer response which ought to come from employing bellows of lower stiffnesses cannot be taken advantage of because of the unstable buckling deflections resulting from compressive mechanical forces introduced to the bellows by the mechanical linkage of the bellows to the vibrating member.
In addition to these buckling deflections resulting in imprecise transfer of mechanical force from the bellows to the vibrating member which produces a significant nonlinearity of response in these known pressure transducers, they can also produce potential early failure modes.
There exists therefore a need for a vibrating member type pressure transducer employing a pressure deformable member without significant stiffness, such as for instance a bellows, to translate a pressure differential .DELTA.p into a directly corresponding change in tensile stress in the vibrating member which can be sensed as a charge in resonant vibrating frequency. But such a transducer must not have the pressure deformable member subject to mechanical compressive forces. A transducer is needed wherein a positive differential in pressure .DELTA.p tends itself to compress the bellows, as opposed to tending to expand it, so that the bellows is effectively under mechanical tension rather than compression when constrained by the vibrating member and its linkage.